170 Report of the State Geologist. 



explanation of these phenomena of the correlative progress of 

 organs, of these modifications parallel and forced, as it were, in 

 distinct groups, of these laws of improvement which are in fine 

 few and constant. 



To sum up a phenomenon in one word is not to explain it, and 

 the objection to this new idea is that it announces in brief terms 

 a known fact, but does not elucidate it. Furthermore, the 

 existence of this force is not constant, since on the one hand we 

 are acquainted with groups in whieh no tendency toward 

 improvement has ever shown itself, while in other cases improve- 

 ment has only manifested itself in a portion of the individuals 

 which have undergone evolution, and again another portion may 

 have existed for long periods without appreciable modifications. 



At the present day we are often obliged, in studying the 

 problems connected with evolution, to confine ourselves to seek- 

 ing through synthetic approaches the enunciation of phenomena, 

 leaving their explanation to a future day when more conclusive 

 facts shall be known, which will throw light on whatever 

 remains obscure in these difiicult questions. 



